Older adults in Sydney who have received one dose of the AstraZeneca vaccine have been urged to get their second dose anytime after six weeks amid the city’s escalating COVID-19 outbreak.

Although the viral-vector vaccine is licensed for a dose interval of between four and eight weeks, the recommended interval between the two doses for optimum antibody response has been 12 weeks.

Advice from the Australian Technical Advisory Group on Immunisation (ATAGI) is that vaccine efficacy progressively dwindles with shorter dose intervals (see box), down to about 60% if it’s a six week interval.

However, the worsening COVID-19 situation in Sydney, where 678 locally acquired cases have been reported since mid-June, has prompted authorities to rethink their advice.